1. Field of the Invention
The present invention relates to an image forming apparatus such as digital copier, printer, facsimile, or plotter, and an optical scanning unit used for such an image forming apparatus. The present invention can be also applied to an optical scanning type barcode reading apparatus, a laser radar for a vehicle, or the like.
2. Description of the Related Art
In conventional optical scanning units, polygon mirrors or galvanometer mirrors are used to deflect beams for writing images. In order to achieve high-resolution high-speed printing operations, the rotational speed of these mirrors has to be increased. However, there is a ceiling to increasing the rotational speed of the mirror because of various reasons, such as limitation in durability of the bearings, heat generation due to air resistance, and noise.
On the other hand, optical deflectors making use of micromachining of silicon have been researched and studied. For example, Japanese Patent Nos. 2924200 and 3011144, respectively, disclose a technique for monolithically and integrally fabricating a vibrating mirror (movable mirror), together with a torsion bar supporting the mirror on its axis, from a silicon substrate. More specifically, Japanese Patent No. 3011144 discloses an example where the vibrating mirror is used instead of the polygon mirror.
In this type, namely the integrally fabricated vibrating mirror with the torsion bar, the size of the mirror surface is small. Hence, one of the advantages of this type is that the reciprocating motion of the mirror is produced by resonance, and that high-speed operation is achieved. In addition, noise and power consumption are reduced because less driving force is required to swing the vibrating mirror.
However, in order to increase the driving frequency in the vibrating mirror, namely to make speed higher, it is necessary to make the torsion bar thick or thin so that the resonance frequency is increased. Therefore, it may not be possible to maintain the sweep angle.
Because of this, in the example using the vibrating mirror, unlike the example using the conventional polygon mirror, it is not possible to scan a wide area. Hence, as disclosed in Japan Laid-Open Patent Application Publication No. 2002-258183, a method wherein plural optical scanning units are arranged where respective main scanning directions are consistent with each other and an image area is divided in the main scanning direction so that image recording is implemented, is utilized.
On the other hand, as a method making speed high without increasing the driving frequency, the image may be recorded in two directions by reciprocating scanning. However, in this case, a scanning starting side is switched between forward scanning and backward scanning and therefore scanning is implemented in zigzags. Hence, the pitch of the scanning line is not constant and this causes unevenness of light and shade. Because of this, image quality is degraded. Hence, an inefficient way for implementing the image recording by using only scanning in a single direction of the reciprocating scanning is utilized.
In Japan Laid-Open Patent Application Publication No. 2002-296534, an example where an irradiation position is moved in a sub-scanning direction as corresponding to the main scanning direction so that the pitch is made constant by correcting the inclination of the scanning line even if the scanned surface is moved, is disclosed.
As described above, in the related art, in a case where the vibrating mirror is used, since image recording is implemented by scanning in a single direction of the reciprocating scanning, there is a trade off between securing a designated sweep angle and the resonance frequency, and there is a limitation to making the speed high.
In addition, in the optical scanning unit using the polygon mirror, it may be possible to make the speed high by increasing the number n of the light emitting sources and scanning in a lump. However, as shown in FIG. 1, even if the light beams from plural light emitting sources are simultaneously scanned in the reciprocating scanning, the period of the zigzags is increased as the number of the light emitting sources is increased. Hence, a dot cannot be formed in a position along a line to be recorded in the backward scanning and therefore unevenness of the light and shade is increased. Therefore, it is not possible to both increase the number n of the light emitting sources and the reciprocating scanning. Hence, there is no method, other than increasing the driving frequency, to be applied to high-speed scanning.